Stinson’s Venerable “Flying Jeep”
Copyright 2021 by James H. Gray
The Stinson L-5 Sentinel is a two-place liaison airplane that was designed shortly before America’s sudden entry into World War II. It was used by the U.S. Army Ground Forces, Army Air Forces, Marine Corps and British Royal Air Force and it mainly served in the campaigns fought in Europe, the China-Burma-India theater, New Guinea, the Philippines and Pacific islands. In small numbers it also saw service in Alaska, South America, Central America and the Caribbean and, of course, it was used extensively in the United States.
The unarmed L-5, or “flying-jeep” as it was informally known, was originally designed to provide short-range liaison, courier and transportation service, but it was typically also used for reconnaissance, aerial photography, artillery adjustment, medical evacuation , search & rescue, and delivering supplies – sometimes performing those missions behind enemy lines. On some occasions it was also utilized for monitoring and controlling troop movements, checking camouflage, dropping leaflets, laying telephone wire, and directing fighter-bombers on close air support (“horsefly”) missions. In a few isolated instances it made aerial attacks using rockets, bazookas and hand grenades or other small explosives. It’s utility on the battlefield was only limited by imagination and the willingness of pilots and their commanders to attempt new things.
In the United States, L-5’s served in liaison and training roles, but it was never used as a primary trainer. Instead it was employed for advanced liaison pilot training that included preparing pilots for combat assignments overseas. L-5’s participated widely in field exercises with ground forces, helped with radio, radar and aerial evacuation training, and generally familiarized ground troops and their commanders with its uses and capabilities. Development of new equipment and tactics was another common role.
Unlike the other liaison types that were civilian planes purchased more or less “off the shelf” and used for primary flight training and artillery spotting, the L-5 was purpose-built and designed to meet the stringent Army-Navy Engineering Handbook standards instead of the less rigorous Civil Aeronautics Authority (CAA) requirements. The only other purpose-built American liaison aircraft that also served during World War II was the O-49 / L-1 Vigilant, also manufactured by Stinson.
The roots of the L-5 can be traced to the pre-war Stinson 105 (aka HW-75), and the follow-on Model 10 Voyager that was evaluated by the Air Corps in 1940 under the designation YO-54, but it was not simply a modified version of those airplanes. An experimental tandem seat version of the Model 10, called the Model 75B (and later V-75C), was developed later in 1940 and looked a lot like the L-5, but the similarity was only skin deep. It shared the NACA 4412 airfoil, slotted flaps, Handly-Page wing slots, and a similar cantilevered main landing gear, also used on the commercial Stinson models, but otherwise the plane purchased by the military was a clean-sheet design that had little else in common with it and no components other than basic hardware were interchangeable. Structurally, the L-5 was in a league of its own, able to withstand more than twice the g-loads the others could handle.
Primarily designed by Athanas P. Fontaine, chief engineer of the Stinson Aircraft Division of the Vultee Aircraft Corporation, with substantial help from aerodynamics expert Professor Peter Altman, Dean of the Aeronautics Department at the University of Detroit, the prototype version of the L-5 was called the Model 76. It was first flown on June 28, 1941 and underwent accelerated service tests with the Army Air Forces in August and September. Garnering a tentative manufacturing contract just days before the Japanese surprise attack on Pearl Harbor, the first production L-5’s did not begin rolling off the assembly line in Wayne, Michigan until November 1942. The long delay was due to an aluminum shortage that necessitated a complete redesign of the wings and tail using plywood and spruce as a substitute and then training workers and re-tooling the assembly line to accommodate the changes.
Prototype Model 76, June 1941
All L-5 production models had wooden wings, empennage and doors covered in cotton fabric finished with butyrate dope. The flying surfaces were painted two-tone green. The fuselage was constructed using heavy-wall chrome-moly steel tubing also covered in cotton fabric. Early wing struts were steel but later ones, beginning in 1944, were made from aluminum. Up until that time the factory was forced to minimize the use of aluminum which was in short supply, so it was only used for the rudder, elevator and aileron frames, the “boot cowl”, engine cowling, tail cone and wing slats, plus the window frames, fuselage fairings and gap seals. Light weight phenolic material was employed for non-structural items such as the instrument panel.
The 1942 redesign of the wings and tail that delayed production for almost eleven months not only released strategic materials for use on other aircraft being manufactured by Vultee, clever engineeering also strengthened those components. Originally engineered to meet a military requirement for the airframe to withstand the strain of repeated 6g pullouts from steep dives — far more than any of the civilian planes adapted for liaison work could handle — the wooden wings were shown capable of sustaining more than +10g’s without failure during static testing at Wright Field. That topped the design strength of the Douglas SBD dive bomber wings and is comparable to the stress limits of modern aerobatic airplanes such as the Pitts S-2 biplane.
The Sentinel was a stout little bird in every aspect but the penalty, of course, was increased weight and that led to somewhat greater takeoff and landing distances and a lower climb rate compared to the Model 76 prototype that had so impressed the military in 1941. However, the trade in performance for strength gave the L-5 unrivaled maneuverability. Liaison pilots loved its ability to perform evasive maneuvers that the comparatively frail L-4 Cub and other liaison aircraft couldn’t manage without risking structural failure. It was also more survivable in a crash due to the heavy construction, particularly so once 4-point restraint harnesses were adopted.
The landing gear of the L-5 was also quite robust, being designed to withstand a 6g vertical drop. The tapered tubular main landing gear legs were damped with long-stroke oleo shock absorbers and the steerable tailwheel assembly was supported on a trailing arm damped with an oleo strut incorporating a coil-over spring. This gave the Sentinel an ability to absorb bone-jarring rough-field landings without back-breaking results. Unfortunately, its Achilles Heel was that the main gear legs were not as resistant to sheering forces as they were to compression, so many a gear leg collapsed after tangling with an unseen gully, drainage ditch, shell hole, or berm. Once damaged in this way, field repairs were difficult to make so many L-5’s were written off after such mishaps.
The six cylinder Lycoming O-435 engine that powered the L-5 had no such frailties. Producing 185 horsepower (190 in the L-5G), it was designed in 1941 specifically for Stinson’s prototype Model 76. This powerplant became renown among mechanics for longevity under combat conditions, and with minimal care it routinely flew more than twice as many hours between overhauls as the 65 horsepower four-cylinder Continental engines used on the Piper L-4 Cub, giving the L-5 a substantially higher in-service ratio. The power of the O-435 gave the Sentinel other advantages over the Cub such as the ability to operate from high elevation mountain airstrips while still carrying an observer and full complement of radio equipment.
Reliability was also a key factor, and pilots implicitly trusted the O-435 to carry them over long stretches of water, jungle and mountainous terrain. As long as an adequate supply of fuel was assured, the O-435 rarely missed a beat. In one instance, using auxiliary fuel tanks placed in their ambulance compartments, a squadron of L-5B’s flew from northern Luzon to Okinawa, a distance of 750 miles over water – an amazing feat for a group of so-called “puddle-jumpers”. That speaks volumes of the confidence placed in the big, low-compression O-435. Its biggest enemy was carburetor ice which is something all normally aspirated aircraft piston engines are susceptible to under the right atmospheric conditions. Enough L-5’s suffered forced landings due to carburetor icing that the use of carburetor heat was eventually mandated for all operations except takeoff when the temperature was below 60 degrees Fahrenheit. A bigger complaint from pilots, however, was the lack of cockpit heat which wasn’t one of the original military design requirements.
PRODUCTION and VARIANTS
Exactly 3,590 Sentinels were delivered under contract by the end of World War II and another 35 to 40 more planes were finished and sold on the civilian market in 1946 and 1947. Altogether, six sub-models were manufactured in two basic styles. The original configuration, shown above, featured a wrap-around “greenhouse” that was designed with artillery spotting in mind. Exactly 1,813 of these “observer” type airplanes were produced under the Army designation L-5, Navy designation OY-1 and RAF name Sentinel I .
A substantial factory modification of the fuselage was implemented in early 1944, featuring a large two-piece rear door allowing a stretcher-bound patient or 250 ponds of cargo to be quickly loaded. The ambulance / cargo version pictured below was produced in five variations including 712 L-5B’s, 200 L-5C’s, 500 L-5E’s, 250 L-5E-1’s, and 115 L-5G’s. The expanded utility provided by the cavernous, box-like fuselage was particularly useful in the campaigns fought on Okinawa, Iwo Jima, in the Philippine Islands and in the Chine-Burma-India theater. However, the more restricted view toward the rear made it less desireable for artillery spotting and more vulnerable to hostile aircraft approaching from below and to the rear. To mitigate that possibility in some combat zones, flying at tree-top level was common practice.
The first production L-5B, christened Sentinel II by the British, rolled off the assembly line in June 1944. The C-model came along in January 1945 and included a mount for a K-20 aerial camera. The E-model premiered in February of 1945 and saw the addition of manually drooping ailerons for improved low-speed handling and slightly shorter takeoffs and landings. The L-5E-1 received larger wheels and tires starting in late May that aided in operations on soft ground. Appearing in July 1945, the L-5G featured a 24-volt electrical system that standardized it with most other military planes. All the previous models had been fitted with 12-volt electrical systems. The G-model also had an improved carburetor and air induction system that increased the engine output from 185 to 190 hp while reducing fuel consumption by an average 1.5 gallons per hour. None of the L-5G’s (called OY-2’s by the Navy) entered service before the war ended, but six years later a number of them served in the Korean conflict.
By modern civil lightplane standards, the performance of the L-5 is considered average and unimpressive, but during World War II it was deemed very respectable. Nominally capable of operating from a 600 foot long dry sod runway at maximum gross weight, when less heavily laden it could easily cope with half that distance in competent hands. During testing at Fort Sill in August 1941, the prototype was consistently able to takeoff and land within 200 feet with just the pilot aboard. However, in normal use the heavier production aircraft faced with a takeoff over a 50-foot obstacle from a rough, unpaved runway needed 800 to 1,000 feet to clear an obstacle. This was considered the minimum safe length for daily operations. On soft ground or in deep grass, the takeoff distance could exceed 1,500 feet to clear a 50-foot obstacle, so in that instance the lightweight L-4 Cub was often superior for getting airborne more quickly. Due to its lower mass the Cub was also easier to stop when landing on icy or muddy short strips where braking was poor to nil. Overall, however, the L-5 was vastly superior to the the Cub and other “grasshoppers”.
The L-5 held some distinct advantages. It could climb at a sustained high angle of attack and virtually hang on it’s 7-foot propeller in slow flight without stalling. With its big slotted flaps lowered 45 degrees it could drop over an obstacle into a short strip in an impressively steep power approach without having to side-slip. On a dry, packed surface, its hydraulic brakes allowed the L-5 to stop in as little as 150 feet with no headwind. These are some of the capabilities that gave the L-5 its legendary but somewhat over-rated notoriety as a STOL airplane. While its short field performance couldn’t compare to the utterly amazing Stinson O-49 / L-1 that it replaced (the production of which was ended in 1942 and later much regretted by many), the Sentinel was deemed an acceptable alternative to larger, far more expensive and more complicated no-compromise Vigilant. All things considered, the “Flying Jeep” exceeded expectations in its appointed role and its reputation as a reliable workhorse was well deserved.
© 2021 James H. Gray / SOPA
[If you quote from this article please give appropriate acknowledgement of the SOPA website and author .]